1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, an image forming method and an image forming apparatus using the electrophotographic photoreceptor, a process cartridge for image forming apparatus using the electrophotographic photoreceptor, and a coating liquid for the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, information-processing systems using an electrophotographic method are making a remarkable progress. In particular, laser printers and digital copiers which record data with light by changing the data into digital signals make remarkable improvements in their printing qualities and reliabilities. Further, technologies used in these printers and copiers are applied to laser printers and digital copiers capable of printing full-color images with high-speed printing technologies. Because of these reasons, photoreceptors are required both to produce high-quality images and to have high durability.
Photoreceptors using organic photosensitive materials are widely used for these laser printers and digital copiers due to their cost, productivity and non-polluting properties. As the organic electrophotographic photoreceptors, the photoreceptors including photoconductive resin typified by poly-N-vinylcarbazole (PVK); charge transfer complex type photoreceptors typified by PVK-TNF (2,4,7-trinitrofluorenon); pigment dispersion type photoreceptors typified by phthalocyanine-binder; and functionally-separated photoreceptors typified by combinations of a charge generation material (CGM) with a charge transport material (CTM) are known.
Among these various photoreceptors, the photoreceptors using organic photosensitive materials are mostly functionally-separated photoreceptors because of having good sensitivity and durability, wherein the charge generation materials and charge transport materials can individually be designed at a molecular level.
A mechanism to form an electrostatic latent image in the multi-layered photoreceptor is as follows:
the photoreceptor is charged and irradiated with light;
the light passes through the charge transport layer (CTL) and is absorbed by the CGM in the charge generation layer (CGL) to generate a charge;
the charge is injected into the CTL at an interface of the CGL and the CTL;
and the charge moves in the CTL by an electric field and neutralizes the charge on the surface of the photoreceptor to form an electrostatic latent image.
However, the photosensitive layers of the organic photoreceptor are easily abraded due to repeated use, and therefore potential and photosensitivity of the photoreceptor tend to deteriorate, resulting in background fouling due to a scratch on the surface thereof and deterioration of density and quality of the resultant images. Therefore, abrasion resistance of the organic photoreceptor has been an important subject. Further, recently, in accordance with speeding up of the printing speed and downsizing of an image forming apparatus, the photoreceptor has to have a smaller diameter, and durability thereof becomes a more important subject.
As a method of realizing high durability of a photoreceptor, methods of forming a protective layer on the outermost surface of the photoreceptor and applying a lubricant thereto; hardening the protective layer; or including a filer therein are widely known. In particular, the method of including a filler in a protective layer is one of effective methods to improve durability of a photoreceptor. However, when a high-insulative filler is included in a protective layer, an electric resistance thereof increases and residual potential remarkably increases. The increase of residual potential is largely caused by the increase of electric resistance due to the filler and an increase of a charge trap site. On the other hand, when an electroconductive filler is used, electric resistance decreases and an influence of residual potential increase is relatively small, but so-called blurred images having fuzzy outlines occurred and an influence on image quality is large.
Therefore, the high-insulative is difficult to use and a low-insulative filler having relatively a small influence of residual potential is conventionally used, and means of equipping a drum heater heating a photoreceptor with an image forming apparatus is used against blurred images caused by the low-insulative filler. By heating a photoreceptor, blurred images can be prevented, however, a diameter of the photoreceptor has to be large so as to be equipped with the drum heater. Therefore, the drum heater cannot be applied to a photoreceptor having a small diameter which is now prevailing in accordance with downsizing of an electrophotographic apparatus, and high durability of a small-diameter photoreceptor is difficult. Further, since an apparatus has to be large to be equipped with the drum heater, the electric power consumption remarkably increases and it takes much time to start the apparatus up, which are of many remaining subjects.
On the other hand, the residual potential increase when the high insulative filler is used causes a high potential of a light portion in an apparatus, resulting in deterioration of image density and tone reproduction. In order to catch up this drawback, potential of a dark portion has to be increased and an electric intensity increases, resulting in not only defective images such as background fouling but also deterioration of a life of a photoreceptor.
As a method of preventing the increase of the residual potential, a method of using a photoconductive protective layer is disclosed (Japanese Patent publications Nos. 44-834, 43-16198 and 49-10258). However, since the protective layer absorbs light and amount of light reaching a photosensitive layer decreases, a problem that sensitivity of a photoreceptor deteriorates occurs, and an effect of the method is slight.
To the contrary, a method of including a metal or metal oxide having an average particle diameter not greater than 0.3 μm in a protective layer is disclosed (Japanese Laid-Open Patent Publication No. 57-30846), by which the protective layer substantially becomes transparent. This method slightly prevents the increase of residual potential, but the effect is insufficient and does not solve the problem. This is because the increase of residual potential when a filler is included in a protective layer is caused by a charge trap due to presence of the filer or dispersibility of the filler more than charge generation efficiency. Even when the filler has an average particle diameter not less than 0.3 μm, a protective layer has transparency if dispersibility of the filler is increased. In addition, even when the filler has an average particle diameter not greater than 0.3 μm, the transparency deteriorates if the filler is agglutinated.
A method of including a charge transport material with a filler in a protective layer is disclosed (Japanese Laid-Open Patent Publication No. 4-281461), by which the protective layer has mechanical strength and the increase of residual potential is prevented. The charge transport material is effectively included in the protective layer to improve charge mobility and to decrease the residual potential. However, if the significant increase of residual potential caused by the filler is due to increase of resistance or charge trap caused by presence of the filler, the method has a limit to improve charge mobility and prevent the increase of residual potential. Therefore, the thickness of the protective layer or the content of the filler has to be decreased, and the method does not satisfy required durability.
As other methods of preventing the increase of residual potential, a method of including a Lewis acid in a protective layer (Japanese Laid-Open Patent Publication No. 53-133444), a method of including an organic protonic acid in a protective layer (Japanese Laid-Open Patent Publication No. 55-157748), a method of including an electron acceptance material in a protective layer (Japanese Laid-Open Patent Publication No. 2-4275) and a method of including a wax having an acid vale not greater than 5 (mg KOH/g) in a protective layer (Japanese Laid-Open Patent Publication No. 2000-66434) are disclosed. These methods improve charge injection at an interface between a protective layer and a charge transport layer and form a resistant portion in the protective layer so that a charge can easily reach the surface. These methods decrease the residual potential, however, tend to cause blurred images and have a side effect of significantly affecting image quality. In addition, when an organic acid is included in a protective layer, dispersibility of the filler tends to deteriorate and the effect is insufficient, and the methods do not solve the problem.
In an electrophotographic photoreceptor including a filler for high durability, in order to realize high-quality images, not only the above-mentioned occurrence of blurred images and increase of residual potential are prevented but also it is important that a charge reaches a surface of the photoreceptor straight without being interrupted with the filler in the protective layer. This is largely influenced by dispersibility of the filler in a protective layer. When a charge injected into the protective layer from a charge transport layer transfers to the surface, the charge is easily interrupted with the filler if the filler is agglutinated, resulting in dispersion of a dot formed of a toner and large deterioration of image resolution. In addition, when light transmittance deteriorates because writing light is scattered by the filler, the image resolution is damaged as well. An influence on the light transmittance also has a close relationship with dispersibility of the filler. Further, dispersibility of the filler largely influences abrasion resistance. When the filler is firmly agglutinated and not well dispersed, the abrasion resistance largely deteriorates. Therefore, in an electrophotographic photoreceptor including a filler for high durability, in order to realize high-quality images, not only the occurrence of blurred images and increase of residual potential are prevented but also it is important to increase dispersibility of the filler in the protective layer.
However, effective means to solve the problems at the same time are not found. When a filler is included in the outermost surface of a photoreceptor, influences of the blurred images and increase of residual potential strongly appear and problems to attain high-quality images still remain. Further, in order to decrease the influences, a drum heater has to be equipped with an apparatus. High durability of a photoreceptor having a small diameter, which requires durability most, is not realized. In addition, this is a large bottleneck against downsizing the apparatus and decreasing the electric power consumption.
Then, the present inventors suggested in EP Patent Application Publication No. 1205808 an electrophotographic photoreceptor, on the outermost surface of which a layer including a filler, a binder resin and an organic compound having an acid value of from 10 to 700 mg KOH/g is formed.
However, the electrophotographic photoreceptor produces lower quality images due a low resistance of the outermost surface although an increase of residual potential can be prevented and a dispersibility of the filler can be improved.
Because of these reasons, a need exists for an electrophotographic photoreceptor having high durability, preventing an increase of residual potential or deteriorated images due to occurrence of blurred images and stably producing high-quality images against repeated use for long periods.